Process and Apparatus for Ablation

ABSTRACT

The invention firstly comprises a method of ablation processing including a step of ablating a region of a substrate ( 1 ) by way of a laser beam ( 3 ) characterised by a further step of removing debris ablated from the region ( 1 ) by way of a flow of a fluid ( 7 ), namely a gas or vapour, a liquid or a combination of these, wherein the flow of fluid ( 7 ) is directed to flow over the region so as to entrap debris and thereafter to remove the entrapped debris from the region by directing the flow of fluid with any entrapped debris away from region along a predetermined path ( 6 ) avoiding subsequent deposition of entrapped debris on the substrate. The invention further comprises apparatus enabling a laser to ablate a region of a substrate characterised by a partially closed debris extraction module (‘DEM’) ( 4 ) located between a focusing or imaging lens ( 2 ) for a laser beam ( 3 ) and a region of a substrate ( 1 ), the DEM ( 4 ) having input ( 8 ) and output ( 6 ) ports by way of which a flow of a fluid (namely a gas or vapour, a liquid or a combination of these) is caused to flow over the region ( 1 ) so as to entrap debris ablated from the region and thereafter to remove the entrapped debris from the region by providing for the flow of fluid with entrapped debris to pass away from region along a predetermined path to prevent subsequent deposition of entrapped debris on the substrate.

This application is a national stage completion of PCT/GB2005/002326filed Jun. 13, 2005 which claims priority from British ApplicationSerial No. 0413029.0 filed Jun. 11, 2004.

TECHNICAL FIELD

This invention relates to a process of, and apparatus for, ablationproviding for removal of a material from a work-piece by means of apulsed laser beam and for control of debris in the form of particles andablation products generated by the process. In particular it relates tothe use of a laser for the deleting, scribing or removal of thin filmsof organic, inorganic or metallic material from large substrates used,by way of examples, for the manufacture of a flat panel display (‘FPD’)or solar panels and laser ablation of complex, dense 3-D structures intoa surface of a large area sheet of polymer to create masters for themanufacture of lens arrays, diffusers and other devices for displayunits.

BACKGROUND ART

Structuring material by a direct process of ablation by pulsed laserbeams is a well established technology used widely for the production ofprecision devices in, but not limited to, the medical, automotive,solar, display and semi conductor industries.

The ablation process involves the exposure of a material surface to oneor more pulses of intense radiation generated by a pulsed laser source.If the laser wavelength is such that the radiation is strongly absorbedin the top layer of the material and the energy density high enough sothat the absorbed energy raises the temperature of the top layer to wellabove the melting point of the material then in this case this top layerof material is decomposed and changed into gaseous, liquid or solidparticle by-products that expand from the surface. The essentialrequirement for the process of ablation to occur is that enough energyis absorbed in the material in a sufficiently short time that thetemperature is raised rapidly to a point such that the materialdecomposes.

For ablating thick materials each laser pulse removes between 50 nm andseveral microns of material depending on the energy density, laserwavelength and material absorption co-efficient. Each pulse behaves inthe same way so that after a succession of pulses fractions of amillimetre of material can be removed. The ablated material is oftenconverted to a gaseous material but in many cases can include bothliquid and solid constituents.

For thin films of material the ablation process can be somewhatdifferent. Where the film is deposited on top of a substrate made of adifferent material and the thickness of the film is low (e.g. less than1 micron) it is possible for ablation to be undertaken by one of twomethods. If the film absorbs the laser radiation strongly then noradiation penetrates to the lower substrate but is absorbed within thefilm. Such strong absorption in a thin layer causes the temperature ofthe film to rise rapidly and heat is conducted to the lower side whereit causes the disruption of the bond between the film and the lowersubstrate. Such a process occurs with thin metal films. In this case themetal is removed in one laser pulse in the form of a mixture ofparticles and liquid.

In the case where the film is wholly or partially transparent to thelaser radiation and the lower substrate absorbs the radiation morestrongly than the film, the energy is absorbed at the top of the lowersubstrate at the interface between the two layers causing a rapidtemperature rise and the ablation of the top layer. In this case theremoved top layer is generally decomposed into particles ranging in sizefrom sub micron to many tens of microns.

If the underlying substrate material is transparent to the laserradiation and the thin film absorbs it then it is sometimes advantageousto bring the laser beam to the substrate/film interface directly throughthe substrate. In such cases the film is often delaminated from thesubstrate in only one laser shot of modest energy density.

All processes of material ablation by laser lead to the generation of arange of ablation product components which can be in gaseous, liquid orsolid form. These include atoms, molecules, clusters, particles, polymerchains, small and large material fragments, liquid droplets and jets andothers. We refer hereafter to this material as ablation debris. Thecontrol of this ablation debris is a significant problem and thedeposition of ablation debris onto the substrate surface has to beminimised to avoid contamination. In particular in the case of thin filmablation for FPD manufacture where the direct laser ablation processreplaces a wet chemical or plasma etching process (in which particulatecontamination does not readily occur) the re-deposition of ablationdebris onto the surface of the substrate during a laser ablation FPDproduction process cannot be tolerated. It is an object of the presentinvention to control the flow of the ablation debris from the substratesurface and minimize it's re-deposition onto the substrate.

Methods have been used before to attempt to capture and control ablationdebris generated during laser ablation processes. Most of these rely onsome type of gas flow near the surface that is being ablated. The flowis often directed along the surface and can be created by blowing on oneside of the area and sucking strongly from the other. The gas used isoften air but in some cases other gases such as helium, oxygen or argonare used. In all cases the flow of gas is used to redirect the movingablation debris and either direct it away from the critical area orpreferably remove it totally from the substrate area. The process relieson momentum exchange between the gas molecules and the ablation debrisand hence high pressures and high gas flow rates are needed for it to beeffective. The use of a heavy gas such as argon can aid this process. Ifhelium is used the effect is different as the mass of helium moleculesis much less than of air molecules, and so helium is less effective thanair in interacting with the ablation debris. In this case the movingablation debris can then travel further from the ablation site beforebeing slowed and deposited. This has the effect of moving the depositedmaterial further from the site of origin but does not significantlyreduce the total amount of material re-deposited.

The use of a reactive gas such as oxygen can reduce the amount ofdeposited material where the ablation debris reacts with the reactivegas to transform it to a pure gas. An example of this is the ablation ofsome polymer materials. Here the organic particles created can reactwith the oxygen to form pure gases such as carbon dioxide or carbonmonoxide.

A liquid flow across a surface is sometimes used as an alternative to agas flow to entrap ablation debris. During the laser ablation process athin layer of water, or other liquid, is directed across the surface ofthe ablation region. The layer is required to be thin so that it doesnot absorb or disturb the incoming laser beam and is generally createdby some type of atomizer nozzle located on one side of the ablationregion. Such a system has been described recently in Clean LaserMachining (Industrial Laser Solutions, May 2003). Having passed acrossthe substrate surface the fluid is collected in some type of channelaround a chuck holding the substrate.

The methods listed above make use of unconstrained gas or liquid flowsdirected across a surface. Such usage is of limited effectiveness inremoving the ablation debris since the capture of the debris is nottotally effective and re-deposition in other areas of the substrateoften occurs. Ablated debris is simply blown or flowed to another areaof the substrate where it re-deposits. Another serious disadvantage ofthe liquid flow method is that it is inappropriate for dealing withlarge substrates associated with FPD manufacture since in this case amounting chuck for the substrate can be very large and any water capturechannels are a long way from the ablation point, As a result there-deposition of ablation debris from the fluid flow onto the substrateis likely.

It is an object of the present invention to avoid these limitations andprovide for removal of ablation debris from the surface of substrates ofany size without significant re-deposition on the surface.

DISCLOSURE OF INVENTION

According to a first aspect of the present invention there is provided amethod of ablation processing including a step of ablating a region of asubstrate (1) by means of a laser beam (3) characterised by a furtherstep of removing debris ablated from the region (1) by means of a flowof a fluid (7), namely a gas or vapour, a liquid or a combination ofthese, wherein the flow of fluid (7) is directed to flow over the regionso as to entrap debris as aforesaid and thereafter to remove theentrapped debris from the region by directing the flow of fluid with anyentrapped debris away from region along a predetermined path (6)avoiding subsequent deposition of entrapped debris on the substrate.

According to a first preferred version of the first aspect of thepresent invention the method of ablation is characterised in that thedirected flow of fluid (7) is constituted by a gas.

According to a second preferred version of the first aspect of thepresent invention or of the first preferred version thereof the methodof ablation is characterised in that the directed flow of fluid (7) iscaused to flow substantially perpendicularly to the region.

According to a third preferred version of the first aspect of thepresent invention or of any preceding preferred version thereof themethod of ablation is characterised in that the directed flow of fluid(7) is caused to flow transverse the region.

According to a second aspect of the present invention there is providedapparatus enabling a laser to ablate a region of a substratecharacterised by a partially closed debris extraction module (‘DEM’) (4)located between a focusing or imaging lens (2) for a laser beam (3) anda region of a substrate (1), the DEM (4) having input (8) and output (6)ports by means of which a flow of a fluid (namely a gas or vapour, aliquid or a combination of these) is caused to flow over the region (1)so as to entrap debris ablated from the region and thereafter to removethe entrapped debris from the region by means providing for the flow offluid with entrapped debris to pass away from region along apredetermined path to prevent subsequent deposition of entrapped debrison the substrate.

According to a first preferred version of the second aspect of thepresent invention the apparatus is characterised in that the directedflow of fluid (7) is constituted by a liquid.

According to a second preferred version of the second aspect of thepresent invention or of the first preferred version thereof theapparatus is characterised by means (4, 6) providing for the directedflow of fluid to flow substantially perpendicularly to the region.

According to a third preferred version of the second aspect of thepresent invention or of any preceding preferred version thereof theapparatus is characterised in that the DEM (4) is closed on a side nearthe lens (2) by a window (5) that is transparent to the laser beam (3).Typically the window (5) has a wiper means for removal of debrisdeposited on the window (5).

According to a fourth preferred version of the second aspect of thepresent invention or of any preceding preferred version thereof theapparatus is characterised in that the DEM (4) is closed on the sidenearest the lens (2) by a plate (12) situated at the stop of the lens(2) that has a hole or an array of holes to allow the beam (13) to passinto the DEM (4) to the region (1).

According to a fifth preferred version of the second aspect of thepresent invention or of any preceding preferred version thereof theapparatus is characterised by the provision of a gap (G′) between theDEM (4′) and substrate (1) to allow the flow of fluid to enter the DEM(4′) to flow over at least part of the region. Typically the DEM (4′) ismounted on a movable slide and the gap provided at the lower edge (5) ofthe DEM (4′) is maintained constant during substrate (1) movement by asuitable substrate surface position sensor linked to the slide.

According to a sixth preferred version of the second aspect of thepresent invention or of any preceding preferred version thereof theapparatus is characterised in that the DEM (4′) is attached to an airpuck that floats on the substrate (1).

According to a seventh preferred version of the second aspect of thepresent invention or of any preceding preferred version thereof theapparatus is characterised in that a flow of fluid through the DEM (4)is created by causing fluid to enter the DEM by means of a pump.

According to an eighth preferred version of the second aspect of thepresent invention or of any preceding preferred version thereof theapparatus is characterised in that a flow of fluid through the DEM (4)is created by extracting fluid from the DEM (4) by means of a pump.

According to a ninth preferred version of the second aspect of thepresent invention or of any preceding preferred version thereof theapparatus is characterised by a gas input port (8) is located in aregion of the DEM (4) off-set from the region (1) to provide for a gasflow for the removal of debris deposited on the window.

The invention provides for substantial benefits in a number of aspectsin the field of ablation. In the gas flow case we believe significantimprovements in the efficiency of the ablation debris removal can beachieved if the gas flow is arranged so that it is directedsubstantially perpendicularly away from the substrate surface ratherthan across the surface. This can be achieved by directing a gas flow inan inwards direction across the surface all around the ablation site andsucking hard above the site. This can be put into practice by theconstruction of a suitable cell located to fill some part of the spacebetween the substrate and the laser beam focusing or imaging lens usedto expose the substrate. This cell is sealed at the top side by a windowthat is transparent to the laser beam and has its lower edge close tothe surface of the substrate. The cell is attached to the device thatholds the lens and hence the substrate can be moved freely below thecell. The cell is partially evacuated by means of a suction pump so thatgas is sucked in through the gap close to the substrate. In this way astrong inwardly directed surface flow is converted into an upwards flowremoving the ablation debris from the surface. If the flow issufficiently intense most of the ablation debris components can beremoved from the surface without any re-deposition. Hereinafter a cellof this general type is referred to as a Debris Extract Module (‘DEM’).

As well as a strong suction connection the DEM can have additional gasentry ports to aid the removal of debris from the substrate and toperform other functions such as prevention of debris depositing on thewindow at the top of the DEM.

Clearly many gases or vapours can be used within a DEM for both inwardflow near the substrate and flow near the window but in many cases anappropriate gas to use for both convenience and cost reasons is air.

A critical aspect of the DEM design is that the distance between itslower edge and the substrate must remain constant at all times even whenthe substrate is moving laterally and the substrate may be uneven, inorder to maintain constant gas flow conditions. Since the focusing orimaging lens also needs to remain a fixed distance from the substrate itis usual to attach the DEM to the same mounting mechanism as the lens sothat both can track the surface of the uneven substrate during motion.Several mechanisms for holding the lens and DEM a constant distance fromthe top of the substrate exist including mechanical, optical, pneumatic,ultrasonic, capacitive and other sensor systems. If such devices areattached to the lower surface of the DEM and the DEM and lens attachedto a servo motor driven slide then feedback of the sensor signal to themotor can be used to maintain the DEM lower edge and the lens distancefrom the substrate constant at all times.

Another method exists for holding the DEM (and lens) a constant distancefrom a substrate. This relies on the use of an air puck such as thosedescribed in our Patent Application PCT/GB2004/001432. In the presentexample the DEM and lens are attached to the top of an air puck that‘floats’ on the substrate surface so maintaining a constant distancebetween them and the surface at all times.

This method has the key advantage that no separate height sensing deviceand servo controlled DEM and lens moving system are needed since thepuck, DEM and lens assembly follow the substrate surface profile at alltimes as the air layer between the puck lower surface and the substratemaintains itself automatically at the same thickness to a high level ofaccuracy. Such a system is of course ideal for the processing of largearea substrates for the manufacture of FPD devices where thicknessvariations of the glass substrate can vary by up to fractions of a mm.

In the simplest case of a DEM attached to an air puck the gas thatenters the cell at the lower side to cause the upward flow past theablation region and entrap the ablation debris is derived from the flowof air that is directed into the channels of the puck to create the airsuspension layer. In this case the fraction of the air escaping in aninwards direction from the lower side of the hollow puck is sucked up bya strong extraction pump attached near the top of the DEM. Such a methodis simple but may be limited in the volume of air that can move to theinside of the puck. This may lead to a less than effective upward flowof air so that the efficiency of the ablation debris extraction islimited.

To overcome this limitation it is proposed that ports are created in theair puck that can be used to direct additional air (or other gas) intothe centre of the hollow puck. The ports are arranged to direct the flowradially inwards from the outside to the inside of the puck and areshaped so that the gas they emit to the interior of the puck is releasedvery close to the substrate surface and is directed at a small angle tothe surface. By this method the gas is caused to flow at high velocityin an inward direction along the surface towards the ablation zone. Asthe gas moves into the hollow core of the puck the suction applied tothe DEM causes the flow to change from radially inwards to upwards somore effectively entrapping the upward moving ablation debris.

It has been found that debris generated by the ablation process can varyin amount and size and removal of some types may be made more effectiveby increasing gas flow for enhanced entrainment of debris. A DEM mountedon an air puck can have additional gas entry and exit ports to improvedebris removal from the substrate and reduce re-deposition of debris onthe DEM window. Ports can either deliver air or gas to the substrate byconnection to a suitable pump or compressor or alternatively gas can beremoved by way of a port connected to a suction pump.

The ports in the puck can be arranged to selectively direct the gas orair flow along the substrate surface in a direction parallel, obliquelyor perpendicular to the substrate motion in a scanning process mode ofoperation. In situations where ablation debris removal efficiency isrelated to the direction of gas flow with respect to the movingsubstrate then this direction can be changed by opening and closingsuitable valves to connect the ports alternately to input or extractflows. Alternatively the whole puck assembly can be rotated to align theports correctly with respect to the substrate motion.

Various other proposed features of DEMs are now discussed. The laserwindow location can be at a variety of different positions. In somecases it may be beneficial to have the window at the top side of the DEMclose to the underside of the lens so that the DEM almost fully occupiesthe space between the lens and the substrate. In other cases it may bebeneficial to situate the window at an intermediate position between thelens and the substrate. The choice of position very often depends on theshape of the laser beam between the lens and the substrate. When thelens focuses the beam the beam size becomes very small close to thesubstrate so that positioning of the window well way from the substrateis important to avoid damage caused by the high laser power. On theother hand when the lens projects a large image and particularly whenthe lens is of telecentric type the position of the window can be closeto the substrate surface without risk of damage by the laser beam.

To allow operation of the DEM for long periods without maintenance it isimportant to prevent deposition of ablation debris within the DEM. Ifsuch an effect occurs deposited debris may fall back onto the substrate.Preferably a DEM should therefore be designed so as to have smoothinterior surfaces without steps, discontinuities or sudden size changes.Such a design of a DEM will allow for an unimpeded flow of gas andminimise the risk of debris deposition within the unit.

Despite these precautions, since the velocity of the gas flow is lowclose to the DEM walls, there still exits the possibility that somedebris may be deposited from the gas flow onto the DEM walls. To preventthis material falling back onto the substrate the DEM is constructed insuch a way that there are no direct paths for debris to follow from theDEM wall to the substrate. Such is achieved by the use of suitablydesigned inversely sloping surfaces or steps.

Since the laser beam has to pass through the window of the DEM it isimportant that the deposition of debris on the window is minimised. Thisis usually accomplished by correct flow of input gas close to the windowbut never the less some deposition may occur. In this case it isimportant to extend the operational lifetime of a DEM by arranging forthe window to be movable so that contaminated parts of the window can beremoved from the beam and replaced with clean areas. Such movements canbe made manually or automatically.

A built in window cleaning system can be provided in order to extend theDEM lifetime. Such a system could be based on a type of moving wiperblade or consist of a static wiper blade across which the contaminatedside of the window can be moved periodically. In yet another version apower- or manually-driven sequence of windows can be provided one beinglocated to function as a working window while the remaining members ofthe sequence are cleaned.

In some cases where the diameter of the laser beam is small and the DEMfills a substantial part of the space between the lens and the substrateit is possible to replace the transparent window with an opaque platewith an aperture or hole. If the aperture is of moderate size oustlarger than the beam size at the plate) and the suction applied to theDEM is sufficient, then an upward flow of gas near the substrate and atthe same time a downward flow of gas through the aperture is created toensure particulate debris is removed from the substrate and in additiondoes not reach the lens.

In this case where the lens is a projection lens and is non-telecentricthen the laser beam forms a focus at a position between the lens and thesubstrate. This position is called the stop. In most cases the beam sizeat this stop point is small and hence a plate placed at this point toseal the top of the DEM needs only a small aperture. In this case theeffect of this hole on the gas flow in the DEM is small.

If a multi-element lens system is used to homogenise the beam before themask (or aperture) that is projected by the lens then the beam at thestop position is no longer a single focal spot but consists of an arrayof focal spots. The number of spots is equal to the number of what areconveniently described as ‘beamlets’ created by way of multi-elementhomogenization optics. Generally this number is in the range of a few to100 or more but any practicable number of spots can be used. In thiscase the plate sealing the top of the DEM has an array of holes ofappropriate size and spacing to allow all beamlets to pass through. Thesize of each of the holes needed depends on the divergence of the laserbeam and the focal length of the lens but for most lasers issubstantially less than 1 mm.

The case where the top of the DEM is sealed with the plate with an arrayof holes located at the stop position most frequently occurs when thelaser is of multimode type such as an ultra-violet Excimer laser or anear Infra-red solid state laser. For these lasers beam homogenizationsystems involving segmentation of the beam into multiple beamlets tocreate a uniform pattern for projection is most commonly used.

In one case air or other gas admitted to a DEM and thereafter extractedfrom the DEM can be contained in a closed cycle flow loop system. Insuch a case it would be convenient to place a pump and filter unit inthe flow loop to trap out the ablated particles. In other cases it maybe more advisable to discharge the air or other gas sucked from the DEMfreely rather than returning it to the DEM. In this case fresh gas orair is supplied to the DEM.

DEMs mounted on air pucks or by other methods to maintain their positioncan be operated in almost any orientation. It is possible to operateDEMs with the substrate vertical and the beam horizontal. Otherorientations are possible including having the substrate at anyintermediate angle between vertical and horizontal.

It is also possible to operate a DEM with the substrate horizontal andthe beam directed vertically upwards from below. When the material to beablated is deposited on a substrate that is transparent to the laserradiation the laser beam can irradiate the film through the substrate ifappropriate. In this case the DEM can be situated on one side of thesubstrate while the laser beam is on the other. In the horizontal casethe beam may come from the top and pass down through the substrate. TheDEM is then below the substrate and captures debris ablated from thelower surface. Alternatively the beam may come up from below thesubstrate and to cause ablation from the upper side with resultingdebris. In this latter case the DEM is on top. In both these cases thelaser beam does not pass through the DEM so no window or perforated beamentry plate is needed.

Liquid Flow Utilisation

Here we propose an improvement arising from the use of a thin layer ofliquid situated on the substrate. In this case this liquid is trappedbetween a window and the substrate at all times. The thickness of theliquid layer does not have to be thin and could fill the entire spacebetween the substrate and the lens but it is expected that moreeffective trapping and removing of ablation debris will occur if thelayer is rather thin in the range of a fraction of a mm up to 1 or 2 mm.

The window is attached to the same mounting arrangement as the lens andboth are held at constant distance from the substrate by a servo motordriven slide activated by a suitable sensor device that detects thelocation of the top of the substrate. The gap between the substrate andthe window thus remains constant at all times. The gap forms a cell orDEM through which liquid is passed to remove particles generated duringthe ablation process. The size of the DEM in the lateral directions isgenerally somewhat larger than the area occupied by the laser beam. Forthe case of an imaged beam the size may be up to 20 mm. For the case ofa scanning optical system the size may be somewhat larger. The DEM shapecan be circular, square, rectangular or any other shape that isappropriate and fits the beam shape at the substrate best.

Liquid is injected into the DEM on one side and extracted on theopposite side so that there is a flow of liquid across the DEM betweenthe window and the substrate. The liquid traps the ablation debris andremoves it from the ablation region.

Such a system is analogous to the methods proposed for optical immersionlithography where the gap between the imaging lens in a wafer stepper orscanner tool is filled with liquid to improve optical resolution anddepth of focus. In this case the substrate is generally a resist coatedwafer and the radiation pattern created by the lens on it exposes theresist which is subsequently developed to form a structure. In this casethe light intensity is very low so no direct ablation of the resistoccurs and hence no ablation debris is generated. In the worst case somegas is liberated during the exposure process. This is entrapped in theliquid and removed.

The invention we propose here is specifically for the case where thelaser beam intensity is sufficiently high to directly ablate materialand form ablation debris. Because of the containment of the liquid bythe DEM window it is likely that this invention will not be appropriatewhere large pressures are generated during the ablation process. Suchwould be the case for high energy worst density irradiation of polymers.If a high pressure is created during the ablation process by gas formedthen this is likely to disrupt the liquid flow and possibly damage theDEM window. Our invention is of particular importance in the case wherethin layers of organic or inorganic materials are caused to be separatedfrom a lower substrate at a modest or low energy density. In this caselittle or no gas is created and no pressure is generated and the liquidflow and cell are not perturbed. Such a situation arises when the thinlayers of materials such as are found in FPDs are patterned by laser.

As the DEM window is close to the substrate clearly this liquid cellinvention is not appropriate for the case where the beam is focussed andis of small size at the window. The invention is appropriate when theimage size is relatively large and the energy density needed to ablatethe thin layer of material is low.

It is possible to envisage a liquid DEM of the type described abovewhere the window is attached directly to the lens and the window (andlens) float on the thin layer of liquid between the window and thesubstrate. This is analogous to the air puck and DEM discussed above butin this case the liquid cell performs both floating and DEM functionssimultaneously.

Both gas and liquid version of the DEM inventions discussed above can beused with any type of laser system with wavelengths ranging from the farinfrared (e.g. 10.6 μm) down to the deep ultra violet (e.g. 157 nm). Themain requirement must be that the optical radiation must be able to passthrough the window material without significant loss and that for theliquid case the liquid must be transparent to the laser radiation. It isgenerally expected that these inventions will be of most use wherelasers in the wavelength range 193 nm to 1.06 μm are used. In this rangefused silica is an ideal window material and water is an ideal liquid.In particular we expect many applications of the liquid DEM device tobecome important where a UV Excimer laser operating at 248 nm or 308 nmis used.

Both gas and liquid forms of the DEM can be operated with the DEMstationary with respect to the substrate or for the cases where there isrelative motion. The stationary case would occur when laser processingis carried out in a step and repeat mode. The moving case will occurwhen the laser processing is in a scanning mode.

BRIEF DESCRIPTION OF DRAWINGS

Exemplary embodiments of the present invention will now be describedwith reference to the accompanying diagrams comprising FIGS. 1 to 10which are diagrammatic views of DEMs.

MODES FOR CARRYING OUT THE INVENTION

FIG. 1 A gas version of the DEM concept where the fluid flow used isair. A flat substrate 1 is irradiated by a laser beam 3 focussed orimaged by lens 2. The laser beam 3 passes through a DEM 4, that isclosed at its upper end by a transparent window 5, and ablates substrate1 in region R. Air, and any entrained debris created by the ablationprocess, is extracted from a port 6 some way up the DEM 4 and isreplaced by incoming air 7 sucked through gap G between lower edge 4A ofthe DEM 4 and the substrate 1. The DEM 4 and lens 2 are maintained atconstant position relative to the substrate 1 by a height sensor coupledto a servo motor driven slide mechanism (not shown).

FIG. 2 A gas-using DEM 4 is provided with an additional port 8 is fittedto an upper region of the DEM 4 to provide for entry of gas to theinterior. This additional port 8 provides for a cleaning flow of air 8Ato be maintained over the underside of window 5.

FIG. 3 A more complex form of a gas using DEM 4 is attached to an airpuck 9 which is fed with air flow 10A through ports 10 to causelevitation of the puck 9 above the substrate 1 so as to maintain gap Gat a predetermined height.

FIG. 4 An air flow DEM 4 is provided with special gas entry ports 11 todirect an additional air flow 11A into the interior of the puck 9, closeto the ablation site R. The ports 11 serve to direct air flow 11A in aninward direction towards the ablation site R and to direct it at someappropriate small angle or as parallel as possible to the substratesurface. The ports 11 can be arranged on two or more sides of the puck 9or can all around the puck 9.

FIG. 5 A DEM 4′ is for use with a substrate 1 which is transparent.Laser beam 3 passes through the substrate 1 to ablate material fromregion R′ on the side 1A of the substrate 1 which is opposite to side 1Babove which the lens 2 is located. In this case the DEM 4′ does not needa laser window.

FIG. 6 A DEM 4″ shows what in FIGS. 1 to 4 was described as a windowreplaced with a plate 12 with an array of holes (exemplified by holesH1, H2) each allowing a beamlet B1, B2 to enter the interior of the DEM4″. The plate 12 is situated at a lens stop where the beamlets 13created by up-stream homogenizer optics are focused by a field lens toan array of focal spots. This is a situation occurring with anon-telecentric projection lens. Only two beamlets B1, B2 are shown inthe FIG. 6 but the number of beamlets and focal spots at the stop can beup to over 100 depending on the number of lenses used in the homogenizeroptics.

FIG. 7 A gas DEM 4 has an internal structure formed so as to minimizethe possibility of debris that might be deposited on the walls of theDEM or fall back onto the surface of the substrate. Ports 6, 6′ arearranged to slope downwards so once inside port 6, 6′ debris is unlikelyto move back towards the interior of the DEM 4. The diameter of the DEM4 increases progressively from the bottom to the top with suitablypumped debris catcher channels 14 located appropriately to catchdeposited debris that might become detached from walls of the DEM 4.

FIG. 8 A gas cell 4 is mounted on an air puck P for use where laser beam3 is horizontal and substrate 15 is mounted vertically. In this case asecond air puck 16 mounted at the rear side of the substrate is used toforce the substrate 15 against the puck P on which the cell 4 is mountedto maintain gap G of constant size.

FIG. 9 This shows a DEM 11 integral with a puck 12. The DEM 11incorporates a box 13 having at upper end 14 a window through which alaser beam L can be directed in the direction of arrow A through the DEM11 and aperture 15 at region 16 of a substrate work-piece 17. Inletducts 18, 19 are provided in puck 12 to enable a gas (in this case air)to be directed into region 20 above that part of the region 16 which thelaser beam L is currently ablating. The air flow is then caused to moveperpendicularly from the region 20 with entrained debris up through duct21 until it enters volume 22 and then passes out of the DEM by way ofoutlet port 23. The dimensions and proportions of the inlet ducts 18,19, region 20, volume 22 and outlet port 23 are chosen to ensure thatthe air flow with the available pressure differentials act to providefor optimisation of the amount of debris removed. In this case ducts 18and 19 provide for directing air flow into the region 20. Howeverprovision is made for flow in one of these ducts to be reversed toprovide for an outflow along that duct from region 20. Further ductscomparable with inlet ducts 18, 19 are provided (though not shown) alongan axis at right angles to an axis common to ducts 18, 19 so that fourinlet ducts spaced at 90° intervals around a periphery of the region 20.

FIG. 10 A liquid flow DEM 4′ is provided with a window 5 close to thesubstrate 1 and a layer of liquid 17 is trapped between the window 5 andupper surface U of substrate 1. The window 5 is attached to the samemechanism that retains lens 2. Both of these are caused to more in acontrolled way with respect to the substrate surface U by means of asensor and servo motor-driven slide device to maintain the spacingbetween the window 5 and the substrate surface U and the lens 2 and thesubstrate U constant. Liquid layer 17 is introduced into the gap G′between the window 5 and the substrate 1 by way of port 18 and extractedby way of port 19.

INDUSTRIAL APPLICABILITY

The present invention provides method and apparatus by means of whichlaser ablation of a region on a work-piece can be readily and accuratelycarried out while debris arising from the ablation is positively removedfrom the vicinity of the region so as to avoid the debris being randomlydeposited elsewhere on the work-piece.

1.-20. (canceled)
 21. A method of ablation processing, the methodincluding a step of ablating a region of a substrate (1) by a laser beam(3) and further comprising the step of removing debris ablated from theregion (1) by a flow of a fluid (7), namely, a gas or vapour, a liquidor a combination of the fluids, wherein the flow of fluid (7) isdirected to flow over the region so as to entrap debris as aforesaid andthereafter to remove the entrapped debris from the region by directingthe flow of fluid, with any entrapped debris, away from region along apredetermined path (6) avoiding subsequent deposition of entrappeddebris on the substrate.
 22. The method of ablation as claimed in claim21, further comprising the step of constituting the directed flow offluid (7) by a gas.
 23. The method of ablation as claimed in claim 21,further comprising the step of causing the directed flow of fluid (7) toflow substantially perpendicularly to the region.
 24. The method ofablation as claimed in claim 21, further comprising the step of causingthe directed flow of fluid (7) to flow transverse of the region.
 25. Anapparatus enabling a laser to ablate a region of a substratecharacterised by a partially closed debris extraction module (‘DEM’) (4)located between a focussing or imaging lens (2) for a laser beam (3) anda region of a substrate (1), the DEM (4) having input (8) and output (6)ports by which a flow of a fluid (namely a gas or vapour, a liquid or acombination of these) is caused to flow over the region (1) so as toentrap debris ablated from the region and thereafter to remove theentrapped debris from the region by providing for the flow of fluid,with entrapped debris, to pass away from region along a predeterminedpath to prevent subsequent deposition of entrapped debris on thesubstrate.
 26. The apparatus as claimed in claim 25, wherein thedirected flow of fluid (7) is constituted by a liquid.
 27. The apparatusas claimed in claim 25, wherein by means (4, 6) providing for thedirected flow of fluid to flow substantially perpendicularly to theregion.
 28. The apparatus as claimed in claim 25, wherein the DEM (4) isclosed on a side near the lens (2) by a window (5) that is transparentto the laser beam (3).
 29. The apparatus as claimed in claim 28, whereinthe window (5) has a wiper means for removal of debris deposited on thewindow (5).
 30. The apparatus as claimed in claim 29, wherein astationary wiper over which the window (5) can be moved to remove debrisdeposited on the window (5).
 31. The apparatus as claimed in claim 25,wherein the DEM (4) is closed on the side nearest the lens (2) by aplate (12) situated at the stop of the lens (2) that has a hole or anarray of holes to allow the beam (13) to pass into the DEM (4) to theregion (1).
 32. The apparatus as claimed in claim 25, wherein theprovision of a gap (G′) between the DEM (4′) and substrate (1) to allowthe flow of fluid to enter the DEM (4′) to flow over at least part ofthe region.
 33. The apparatus as claimed in claim 32, wherein the DEM(4′) is mounted on a movable slide and the gap provided at the loweredge (5) of the DEM (4′) is maintained constant during substrate (1)movement by a suitable substrate surface position sensor linked to theslide.
 34. The apparatus as claimed in claim 25, wherein the DEM (4′) isattached to an air puck that floats on the substrate (1).
 35. Theapparatus as claimed in claim 25, wherein a flow of fluid through theDEM (4) is created by causing fluid to enter the DEM by means of a pump.36. The apparatus as claimed in claim 25, wherein a flow of fluidthrough the DEM (4) is created by extracting fluid from the DEM (4) bymeans of a pump.
 37. The apparatus as claimed in claim 25, wherein a gasinput port (8) is located in a region of the DEM (4) off-set from theregion (1) to provide for a gas flow for the removal of debris depositedon the window.
 38. The apparatus as claimed in claim 25, wherein the oreach input port provides for an incoming flow of fluid to be enabled toflow in an inward radial direction towards the region.
 39. The apparatusas claimed in claim 25, wherein the interior of the DEM is smooth andfree of discontinuities affecting the flow of fluid.
 40. The apparatusas claimed in claim 25, wherein the flow of fluid is caused to flow in aclosed loop, and the fluid on extraction from the DEM is re-circulatedand returned to the DEM.